Using Opus for about 12 years now.
Recently added "Anker" USB 3.0 PCIe card to Win7 desktop.
File transfers SATA HD to 3.0 SanDisk 64GB flash drive will not complete properly.
Identical transfers complete perfectly using Win7 File explorer.
Multiple attempts produce same results.
Make sure the drivers for the USB3.0 card are the latest versions, and that all Windows updates are installed.
Preferences / Miscellaneous / Advanced: copy_nonbufferio_threshold should be set to 0 by default, but if it is not please try setting it to 0 (zero) to see if that fixes it.
Setting Preferences / Miscellaneous / Advanced: copy_buffer_size to 64 KB or 32 KB may also be worth a try. (Change will take effect after restarting Opus. Exit when Prompted via Preferences, or fully exit Opus (via File / Exit Directory Opus) if you want to exist afterwards manually. Do not just close the window and open a new one, as that will not normally exit the program.)
Anti-virus scanners may also get involved, and prevent some processes writing to USB devices, perhaps.